The boom and arm member of an excavator or other construction machine is constantly subjected to repeated load. It is therefore necessary to give sufficient consideration to the safety of weld zones against fatigue during operation of the construction machine.
In general, the fatigue strength of a weld zone is remarkably lower than the fatigue strength of the matrix material. The main reasons are known to be of the stress concentration at the weld toe, the formation of tensile residual stress sites at the weld toe, and the growth of the crystal grains of the weld heat affected zone.
As measures against this, in the past the method of grinding the weld zones to prevent the concentration of stress at the weld zones and the method of placing the welded boom and arm member in a heat treatment furnace for post-heat treatment in so-called “SR treatment” to reduce the tensile residual stress on the weld zones have been used.
However, grinding has the problem of poor work efficiency and further of its effect being largely governed by the skill of the operator, e.g., with over grinding of the weld zones resulting in a reduction in the joint strength.
Further, SR treatment has the problem that it requires a large sized heat treatment furnace and involves tremendous capital costs and operating costs.
Further, Japanese Patent Publication (A) No. 2003-113418 proposes the method of pretreating locations of a metal material where fatigue would become a problem, then treating it by ultrasonic shock waves and further then inspecting it for quality assurance so as to improve the fatigue life of the metal material and discloses that ultrasonic shock wave treatment causes the weld toes to deform with curvature and the degree of stress concentration to change.
Further, Japanese Patent Publication (A) No. 2004-130313 proposes a method of improving the fatigue strength by striking the vicinity of the top parts of weld toes of fillet weld joints, where two overlapping ends are welded together, by an ultrasonic vibration terminal.
However, in these proposals, the case of applying ultrasonic striking treatment to the boom and arm of a construction machine and the conditions for the same were not studied.